allarch recipes using cmake broken (if allarch inherit is after cmake)

Hello,

since this change:
cmake.bbclass: Make it work with ccache
https://patchwork.openembedded.org/patch/157226/

the allarch recipes are no longer allarch, because they contain arch
specific CC and CXX (OECMAKE_C_COMPILER, OECMAKE_CXX_COMPILER variables).

e.g. one example where I've initially noticed this:
Hash for dependent task luna-webkit-api/luna-webkit-api.bb.do_package
changed from
0cc5f187fc5585a39c30ba78a14c0483699e155fb018e339404b679fc45d9dea to
6fdcb8b887c629d4c90c8e09d00a6eefead643f58a77b521af7abde2f354433f
  Hash for dependent task luna-webkit-api/luna-webkit-api.bb.do_install
changed from
606366f8c6ebddaf188803b3df1b4f1e2e531baddda484d34c40e5cd15c848d0 to
115a62259de2a0442c0c8249cadea44f29a21bfabe33563c7ac63ebf3023abec
    Hash for dependent task luna-webkit-api/luna-webkit-api.bb.do_compile
changed from
028e37300d041ebb67229f9b598db7e911fb4c442d6a184246b22bc579f69efb to
d76b16b6ffd6350188ebc4889b7939314a26981b8a06c976ed07d690d52f214d
      Hash for dependent task
luna-webkit-api/luna-webkit-api.bb.do_configure changed from
b64f3e5e2cf9a11d86337031322bc5a5eaea11b92889ff39edf0f545e8168eaf to
9f8713644b6400252935e026082cd6f2e9c73463e5c3aef56b132912f1c0adc9
        Hash for dependent task
luna-webkit-api/luna-webkit-api.bb.do_generate_toolchain_file changed from
d4a00eebc853da4e5401895fa8098e126110fc47d41b408853c0f6c6928a831c to
a32308b26aa0c403cee4da2a243b1b95177f3b08b9ce9eb02d64aca93064cd53
          basehash changed from
daf462705e18f12c238b2f7d6e962cb6479fbc543065ed359b47978797271645 to
7dce39db5e776de0ab31227ab50f5d6aae289777a6a75d2df7aceeb71e370249
          Variable OECMAKE_C_COMPILER value changed from
'arm-webos-linux-gnueabi-gcc' to 'aarch64-webos-linux-gcc'
          Variable OECMAKE_CXX_COMPILER value changed from
'arm-webos-linux-gnueabi-g++' to 'aarch64-webos-linux-g++'

Reverting this change resolves the issue.

Setting empty CC/CXX in allarch.bbclass fixes this issue as well, but only
when allarch is inheritted before cmake.

Actually it looks like moving allarch before cmake inherit helps even
without setting CC/CXX empty there.

Regards,